The invention relates to a method and a driver circuit for the transmission of differential signals.
For the transmission of digital data by electric signals, differential busses, for instance, the CAN bus known from the automotive field, are frequently used.
In a differential bus, the digital signal to be transmitted is transmitted on a transmission line, and the complementary signal on a second, parallel line. The transmission by a differential bus offers advantages vis-à-vis a single-ended transmission in which only the signal is transmitted via a line and the receiver uses GND as reference potential, or a transmission with one signal and one reference potential line. Perturbations by foreign electromagnetic fields influence both transmission lines in equal or similar manner during the differential transmission. Since the difference of the transmitted signals is generated at the receiver side for retrieval of the desired signal, the perturbations are at least partially eliminated, so that the differential transmission of a digital signal is less sensitive to perturbations by foreign fields.
Another advantage of a differential bus is the minor radiation of electromagnetic fields that may have an interfering effect on other data transmission lines. In the ideal case, i.e. if the signals are exactly complementary to each other, no electromagnetic field is radiated by the conductors altogether since the field radiated by a conductor is exactly complementary to that of the other conductor, and they therefore cancel each other out.
In differential bus systems, the two signals are, however, frequently not ideally complementary to each other, so that undesired fields are radiated. Thus, in the case of not ideally complementary signals there results a common-mode portion that causes the radiation.
The greatest voltage changes as well as peaks in the current flow are caused by an edge in a digital signal. Here, it has to be taken into account that, in an ideal system, a signal edge has an infinite rise, but that in practice, due to the non-ideal characteristics of electronic devices, the rise of a signal edge is finite. Thus, the intensity of an electromagnetic field radiated by the conductors of a differential bus depends, in addition to the simultaneousness of the signal edges, in particular on the edge steepness of the signals.
For reducing or for preventing the radiation of an undesired electromagnetic field by the conductors of a data transmission path, the bus drivers feeding the signals that are complementary to each other into the lines therefore have to be designed such that they generate signals having an edge steepness as equal as possible. For the design of circuits by which the edge steepness can be influenced, it has to be taken into account that, for the bus driver of the high level signal, e.g., P-MOS transistors are used, and for the bus driver of the complementary signal e.g., N-MOS transistors.
For the design of circuits for controlling, fuses are used in accordance with prior art, i.e. thin, metal conductor paths that are adapted to be cut through by a laser and by which a trimming of the bus drivers can be performed once only during manufacturing.
U.S. Pat. No. 5,886,554 describes a circuit of a driver which enables the controlling of the edge steepness, wherein the edge steepness of the rising signal edge is adapted to be controlled independently of the edge steepness of the falling signal edge. The edge steepness is respectively controlled in that a current is conducted through a capacitance, so that the voltage and thus the edge steepness of the voltage rise may be controlled via the current intensity and the size of the capacitance. By a switch switched in the cycle of the signal to be transmitted, the current flow is conducted, at the rising edge, through a first, and at the falling edge through a second capacitance, which may be dimensioned irrespective of each other, and thus the respectively separate controlling of the edge steepness is enabled. For controlling the edge steepness of the signal and of the complementary signal, the described circuit is connected in the signal path of the signal and in the signal path of the complementary signal.
These approaches for trimming the steepness of the signal edges with those of the complementary signal have the disadvantage that the dimensioning of the devices used is determined once only, or a trimming can take place once only. Fluctuations in the characteristics of the devices controlling the steepness of the signal edges, which are, for instance, caused by aging or temperature fluctuations, thus cannot be compensated for during the operation of the drivers.
For these and other reasons, there is a need for the present invention.